Machines such as, for example, wheel loaders, on and off-highway trucks, motor graders, and other heavy construction and mining machines are used to perform many tasks. To effectively perform these tasks, the machines require a power source that provides significant power through a transmission to one or more ground engaging devices. The transmission must provide a range of gearing in order to allow the machine to work at different speeds while keeping the engine operating within a desired operating range. For this purpose, the machines typically include a multi-speed transmission connected to the engine via a torque converter.
To generate the wide range of gearing required by the machine, the multi-speed transmission includes a plurality of intermeshing gears and a corresponding shift mechanism also commonly known as a shift yoke or rod. Each of the gears have different numbers of teeth and the output gear ratio or speed of the transmission depends on the combination of engaged gears. The shift mechanism is used to selectively engage predetermined combinations of gears that result in a desired output ratio. For example, the shift mechanism is typically movable between three positions, namely from a first position at which a first combination of gears is selected to produce a first output gear ratio (e.g., high speed), to a second position at which no gears are engaged (e.g., neutral), and to a third position at which a second combination of gears is selected to produce a second output gear ratio (e.g., low speed).
Shift mechanisms of the type described above are typically hydraulically actuated to move between the three positions. Specifically, each shift mechanism includes a first hydraulic actuator located at one end, and a second hydraulic actuator located at a second end. To initiate movement of the shift mechanism to the first position, the first hydraulic actuator is filled with a pressurized fluid while the second hydraulic actuator remains empty or is drained of the pressurized fluid. To initiate movement of the shift mechanism to the third position, the second hydraulic actuator is filled with a pressurized fluid, while the first hydraulic actuator remains empty or is drained of the pressurized fluid. To initiate movement of the shift mechanism to the second position, pressurized fluid may be drained from both the first and second hydraulic actuators, allowing one or more biasing springs to move the shift mechanism to the second position. Unfortunately, because of the multiple actuators located at both ends of the shift mechanism and because of the use of biasing springs, this configuration is complex, expensive, and unreliable.
One attempt to simplify the shift mechanism design described above and improve transmission reliability is described in U.S. Pat. No. 6,484,600 (the '600 patent) to Bennett et al. In particular, the '600 patent describes a transfer case having an actuator located at only one end of a shift rod. The actuator consists of a pair of pistons, including a connected piston (e.g., a piston fixedly connected to the shift rod) and a free piston (e.g., a piston free to slide along the shift rod). The pair of pistons divide a bore into three separate chambers selectively supplied with compressed air via three separate fluid connections to move the shift rod between high speed, low speed, and neutral. The opposing end of the shift rod is free to move within a blind bore.
In order to achieve high speed, compressed air is supplied to the first two of the three chambers, but not to the third. This causes the fixed piston, free piston, and shift rod to move in one direction until the shift rod reaches a stop and the correct combination of high speed gears are engaged. To achieve low speed, compressed air is supplied to the third chamber, but not the first two. This causes the fixed piston, free piston, and shift rod to move in a second direction opposite the first until the shift rod again reaches a stop and the correct combination of low speed gears are engaged. To achieve neutral, compressed air is supplied to only the second chamber. This causes the fixed piston and shift rod to move to a position midway between the two stopped positions where no gears are engaged.
Although the transfer case of the '600 patent may have fewer components than the typical two actuator design, it may still be complex, expensive, and unreliable. Specifically, the transfer case still requires too many fluid supply connections and, because a valve element must be associated with each supply connection to selectively control the flow of compressed air thereto, the component cost of the transfer case may be excessive. Further, these additional components increase the control complexity and decrease the reliability of the transfer case. In addition, because the neutral position is achieved based solely on a balance of pressure without any hard stops, malfunction (e.g., the undesired engagement or partial engagement of gears) may be possible.
The disclosed transmission is directed to overcoming one or more of the problems set forth above.